A compound semiconductor is a semiconductor composed of a plurality of elements. For example, compound semiconductors containing silicon carbide, gallium nitride, gallium arsenide, or the like have been known. Compound semiconductor substrates containing silicon carbide or gallium nitride are used as materials for manufacturing LEDs or power devices. When the compound semiconductor substrates are used for various materials, it is required to polish the substrate surface into a super-smooth surface. Various polishing compositions have been used for polishing the surface of such compound semiconductor substrates (for example, see Patent Documents 1 and 2).
Silicon carbide and gallium nitride have an excellent corrosion resistance to, i.e., high chemical stability in, acids and alkalis and have the second highest hardness after diamond. Therefore, polishing of compound semiconductor substrates containing silicon carbide or gallium nitride requires a large amount of time. Accordingly, what is important for the polishing compositions used for polishing the above-described compound semiconductor substrates is realization of a high polishing rate.
In Patent Document 1, an oxidizing agent such as orthoperiodic acid or metaperiodic acid, and colloidal silica abrasive grains are contained in a polishing composition for polishing a silicon carbide substrate, whereby the polishing rate is intended to be increased. In Patent Document 2, an oxidizing agent such as tungstates or molybdates, an oxygen donor, abrasive grains, and a pH adjuster are contained in a polishing composition, whereby the polishing rate is intended to be increased. Specifically, when the oxidizing agent is a transition metal salt, the valence of the oxidizing agent changes in the oxidation reaction since the transition metal element can have a plurality of oxidation numbers. In addition, the oxygen donor restores the changed valence of the oxidizing agent, whereby the high oxidative power of the oxidizing agent can be maintained. In the polishing composition, use of the oxygen donor and the transition metal oxidizing agent in a proper combination leads to an increased polishing rate.